In conventional electrophotography, a photoconductive surface is charged in the dark and then subjected to a light image of an original which is to be reproduced. This generates a latent electrostatic image corresponding to the original, which may be a document or a photograph. The latent electrostatic image is made visible by toning with charged pigment or pigmented particles. The most widely used photoconductor in electrophotographic machines is amorphous selenium which has been corona-charged so that the latent image is formed with positive ions. The toner particles for selenium photoconductors must bear a negative charge, and images may be toned by dry particles which adhere to the latent image and are then fused after transfer to a carrier medium. The toner particles may be disseminated through an insulating carrier liquid so that the toner particles will travel to the latent image by electrophoresis. In the copending application of Mooney et al, referred to above, it is pointed out that selenium has many disadvantages, such as its limited spectral sensitivity range and its poor wear characteristics.
The prior art has recognized the advantages of polycrystalline cadium sulphide as a photoconductor. Unfortunately, the prior art (Kuehnle U.S. Pat. No. 3,884,787) has been unable to form a photoconductor of cadmium sulphide of adequate thickness to create a sufficiently high voltage on the surface of the photoconductor with the required charge densities. As a result, the development of a latent image produced on the surface of the photoconductor will require an inordinately long time period for its development to enable it to be practical. Cadmium sulphide, as an electrophotographic conductor, has several disadvantages. First, it has a memory; that is, after forming a latent electrostatic image, developing that image, and transferring it to a carrier sheet, a portion of the latent electrostatic image remains on the photoconductor. Stated otherwise, the decay time in the presence of light is too slight, so that offsetting occurs. Another disadvantage of cadmium sulphide as an electrophotographic conductor is fatigue; that is, as the photoconductor is used and reused, the maximum voltage to which it can be charged becomes less and less. Cadmium sulphide, however, can retain a much higher charge density than tellurium-doped selenium. Chiefly, cadmium sulphide, in addition to being harder, has a wider spectral photosensitivity than selenium. The photoconductors of the prior art are deficient in that their dark decay is too high. Many advantageous photoconductors cannot be used, owing to their rapid dark decay.
Although our multilayer electrophotographic receptor is described in terms of a conventional electrophotographic process, it is also useful in other modes, such as charge-transfer electrophotography (Walkup, U.S. Pat. No. 2,825,814).